The present invention relates generally to a method and apparatus for molding using an expandable mold.
Conventional methods of molding typically utilize two-piece molds. When the separate halves of a two-piece mold are mated and sealed, a liquid material can be injected into the hollow space between the halves. The contours inside this hollow space define the shape of the final molded article. The liquid is cured, and when it is sufficiently hardened, the halves may be de-coupled to facilitate removal of the finished article from the mold. The halves of the mold are typically made of rigid materials to withstand the pressure and heat present in the injection and hardening process and to resist deformation during the molding process. Resistance to deformation is particularly important when the tolerances for the molded article are small.
Molding using such conventional techniques often results in a build-up of excess material on the molded article coincident with the seam between the two halves of the mold. This is especially true when highly detailed articles are to be molded since in such cases the entire molding process is geared towards accurate reproduction of very small features. Unintended but unavoidable small gaps that exist at the seam between the two halves of the mold are thus reproduced along with the desired features of the mold.
A method of seamless article molding is disclosed in U.S. Pat. No. 3,841,822 to Putzer et al. There, a generally hollow, one-piece, distensible mold was provided. The inner surface of the mold is formed to give the molded article its final shape. The walls of the mold are designed to be thick enough to support the weight of the material poured into the mold without distortion, and yet thin enough to remain flexible. When the molded article is finished, the mold can be radially expanded by applying a pressure differential between the inside and outside of the mold. This is done either by placing the mold in a vacuum pot to create a low pressure outside the mold or by injecting fluid into the mold to create a high pressure inside the mold. Using either method, the mold is expanded sufficiently to remove the finished article.
The Putzer mold, while allowing seamless molding of fairly complex shapes, addresses only the problems with reproducing macroscopic details from molds to molded articles. However, there are applications where it would be beneficial to replicate molded parts having detailed features that must be reproduced down to a microscopic scale without concurrently imparting seam lines on the article. For example, it may be desirable to fabricate an illumination device that is engineered to provide a precise pattern of light at precise intensities at a location that is remote from a light source.
Fabricating such an illumination device may begin by forming a transparent elongated fiber core. The fiber core is designed such that light that is injected into the fiber at one end travels to the other end without loss of light due to transmission at the surface of the fiber. This well-known phenomenon is called total internal reflection. As taught in U.S. Pat. No. 5,432,876 (Appledorn et al.), features may be imparted onto the surface of the fiber that allow a controlled xe2x80x9cextractionxe2x80x9d of light through the walls of the fiber. Appropriate design of such features will produce a precise pattern of extracted light.
Various methods of producing such extraction structures are known. One method involves micro-machining such structures into a tape with an adhesive backing that can be adhered to the fiber core. However, in this manner, two extra interfaces are added, namely the fiber/adhesive interface and the adhesive/tape interface. Each such interface will reduce the precision of light extraction. Another method is to micro-machine the features directly into the fiber itself. Among the problems with this solution is that materials having desirable fiber properties (e.g. transparency, flexibility, high refractive index) often are not amenable to precise micro-machining. Thus, it may be very difficult, costly, and time-consuming to directly micro-machine features with a microscopic degree of precision and accuracy into fiber core materials.
The present invention provides a method and apparatus for microreplication using a one-piece expandable mold to thereby produce molded articles having microstructured surfaces and having no molding seams.
In one aspect, the present invention provides an apparatus for seamless microreplication using an expandable mold. The apparatus includes an expandable one-piece mold having a generally hollow portion of flexible material. The hollow flexible portion of the mold includes an inner surface and an outer surface. The inner surface is characterized by a plurality of microstructured features and is accessible through an opening in the outer surface of the hollow flexible mold. The apparatus also includes a means for expanding the flexible mold.
In another aspect, the present invention provides a process for making a one-piece expandable mold for seamless microreplication. The process begins with providing a stock material which is machined to form a mold master. During machining of the mold master, a plurality of microstructured features are imparted into the surface of the mold master. The mold master is then covered with a curable material that, when cured, forms an expandable mold. Also, during the curing process when the material is hardened, the material is imparted with a replica of the plurality of microstructured features from the mold master so that the inner surface of the mold contains the microstructured features. Finally, the expandable mold is separated from the mold master by expanding the mold relative to the mold master and removing the master from the mold.
In still another aspect, the present invention provides a method for seamless microreplication using a one-piece expandable mold. First, an expandable one-piece mold is provided. The one-piece mold has an inner surface characterized by a plurality of microstructured features. The one-piece mold is then filled with a curable material which is hardened to form a molded article. The step of curing the curable material substantially replicates the microstructured features of the inner surface of the mold into the surface of the molded article. Once formed and hardened, the molded article may be removed from the mold by elastically expanding the mold. The step of elastically expanding the mold preferably takes place by injecting a fluid under pressure between the mold and the molded article.
In a particular embodiment, the method for seamless microreplication using the one-piece expandable mold of the present invention may be employed to make an illumination device. In so doing, a curable material is chosen such that, when hardened, it forms a substantially optically transparent article. In addition, the microstructured features of the inner surface of the mold are designed such that, when replicated into the surface of the molded article, the microstructured features form a plurality of light extraction structures on the transparent molded article. The mold is then elastically expanded to remove the illumination device.